Nanostructured sensors for molecular recognition

A survey is given on typical ‘top-down’ and ‘bottom-up’ approaches to design nanostructured sensors which monitor different physical and chemical quantities. Particular emphasis is put on new materials and transducers for molecular recognition by chemical sensors. These convert chemical information into electronic signals by making use of suitable ‘key-lock’ structures. This requires the control of surface structures of chemically sensitive materials down to the molecular scale under thermodynamically or kinetically controlled conditions. This in turn requires the molecular understanding of sensor mechanisms which is deduced from comparative microscopic, spectroscopic and sensor test studies on ‘prototype materials’. Selected case studies illustrate the common mechanisms of molecular recognition with electron conductors, ion conductors, mixed conductors, molecular cages, polymers and selected biomolecular function units.

Molecular manufacturing: perspectives on the ultimate limits of fabrication

Molecular machine systems, common in biology, can become a basis for a new style of physical technology. Characteristic features of the proposed systems and their products include nanometre-scale structures, atomic precision, low defect densities, and high manufacturing productivity.

Micromechanisms

Micromechanics deals with micromechanisms which fall into two broad categories: sensors and actuators. Since sensors measure some property of their environment, internal sensor power dissipation should be minimized and sensor sensitivity must be maximized. In force sensing, power dissipation has been reduced by ten decades in twenty years. Sensitivity has been increased by twelve decades and is now being limited by thermal noise problems. Practical force sensing via mechanically resonant devices, which can be powered by unmodulated light and sensed by optical reflections, has been demonstrated and has major implications on future sensing systems. Actuators are devices which do work on their environment. The tool to produce microactuators is still a major problem. X-ray-assisted processing with very large structural heights satisfies most of the tool requirements for microactuators. It has been used, along with assembly, to produce magnetic actuators, such as rotational motors, with 120 µm rotors and rotational speeds of up to 150 000 rpm. A generic linear electrostatic actuator with large travel and large output force per unit chip area addresses practical markets for this evolving technology.

High- T c superconductors tailored on the nanometre-scale

High- T c superconductors are characterized by an unusually small coherence length, which amounts to a few angstroms only. As the coherence length is the length scale in which a superconductor has to be structured to achieve Josephson junction behaviour, considerable effort has been devoted by many groups to modify high- T c films in the nanometre scale. Because the high- T c cuprates do not lend themselves for nanostructuring, new concepts have to be developed to achieve this goal. These developments will be discussed and an overview of the state of the art of the field will be presented with a special focus on the ultimate limitations of nanoscale structuring of superconductors.

Nanotips and nanosources: application to low-energy-electron microscopy

The characteristics of e-beams held emitted from nanotips, which are atom-sources of electrons, were analysed experimentally for non-magnetic and ferromagnetic nanotips. Their specific characteristics are fully exploited in a versatile low-energy electron projection microscope: the Fresnel projection microscope. Observations of nanometric fibres of carbon and of organic materials were performed with working voltages around 200 V. The images show, in the direct space, details of the objects of less than 1 nm without any magnetic shielding.

Fabrication limits of electron beam lithography and of UV, X -ray and ion-beam lithographies

The paper discusses and compares the lithography methods being developed for the fabrication of future generations of silicon integrated circuits. The smallest features in today’s circuits are about 0.3 μm in size and this will be reduced to 0.1 μm within the next ten years. The methods discussed include optical (ultraviolet light) projection, which is used predominantly at present, projection printing at wavelengths between the X-ray and ultraviolet regions, X-ray proximity printing, and scanning and projection with electrons and ions. There are severe problems to be overcome with all of the methods before they can satisfy future needs. The difficulties are not just connected with obtaining adequate resolution. The more challenging requirements are those associated with the elimination of distortion in the highly complex trillion pixel images and of achieving an exposure rate of about one per second with a system of acceptable cost, that is less than about $10M. The various approaches for correcting distortion and obtaining adequate throughput are described, as are the factors limiting resolution. Finally, the ultimate capabilities of electron beam methods for fabricating structures and devices with dimensions down to 1 nm are described.

Nanocrystals and nano-optics

Three dimensional electronic quantum confinement in semiconductor nanocrystals, and near-field optical spectroscopy of single molecules, are briefly discussed as examples of new science and technology at the nanometer scale.

Is quantum mechanics useful?

Technologies differ in their explicit utilization of quantum mechanical behaviour. A transistor, despite its roots in energy band structure, does not invoke quantum mechanically coherent transmission between terminals. The impressive progress in the past decade in mesoscopic physics, when combined with studies that have analysed a totally quantum mechanical computational process, suggest that we may be ready to move toward more quantum mechanical procedures for information processing. This paper is a warning signal; this possibility is beset by problems. The case will be made via two separate but complementary arguments. First, by summarizing this author's published comments on computation via totally quantum mechanical coherent Hamiltonians. The computation is likely to suffer from localization , i.e. from reflection of the computational trajectory, causing the computation to turn around. Additionally, small errors will accumulate and cause the computation to go off track. This is supplemented by analysis of specific proposals that suggest more detailed machinery than invoked in the general literature on quantum mechanical Hamiltonian computation.

The Narasimhan—Seshadri theorem for parabolic bundles: an orbifold approach

Given a stable parabolic bundle over a Riemann surface, we study the problem of finding a compatible Yang-Mills connexion. When the parabolic weights are rational there is an equivalent problem on an orbifold bundle. When the weights are irrational our method is to choose a sequence of approximating rational weights, obtain a corresponding sequence of Yang-Mills connexions on the resulting orbifold bundles and obtain the solution as the limit of this sequence: we need to consider mildly singular connexions which locally about a marked point take the form d — Aid# + a . Here A is a constant diagonal matrix whose entries depend on the weights and their rational approximations, 0 = arg(z) for z a local uniformizing (orbifold) coordinate centred on the marked point and a is an L 2 1 connexion matrix. In this context we find all the necessary gauge-theoretic tools to prove the theorem, including a version of Uhlenbeck’s weak compactness theorem, provided | A| is sufficiently small. (One of the advantages of this approach is that we do analysis on a compact orbifold rather than on the punctured surface.) Our methods also allow us to consider the analogous problem for stable parabolic Higgs bundles.

Rigorous link between the conductivity and elastic moduli of fibre-reinforced composite materials

We derive rigorous cross-property relations linking the effective transverse electrical conductivity cr* and the effective transverse elastic moduli of any transversely isotropic, two-phase ‘fibre-reinforced’ composite whose phase boundaries are cylindrical surfaces with generators parallel to one axis. Specifically, upper and lower bounds are derived on the effective transverse bulk modulus k* in terms of cr* and on the effective transverse shear modulus //* in terms of cr*. These bounds enclose certain regions in the ct*-ac* and cr*-/r* planes, portions of which are attainable by certain microgeometries and thus optimal. Our bounds connecting the effective conductivity cr* to the effective bulk modulus ft* apply as well to anisotropic composites with square symmetry. The implications and utility of the bounds are explored for some general situations, as well as for specific microgeometries, including regular and random arrays of circular cylinders, hierarchical geometries corresponding to effective-medium theories, and checkerboard models. It is shown that knowledge of the effective conductivity can yield sharp estimates of the effective elastic moduli (and vice versa), even for infinite phase contrast.